New Zealand Needs a Constitution!
Of all the sovereign nations on the face of the planet, there are only three which do not have a single document supreme law that overrules the politicians of the day: The UK, Israel and New Zealand.
Those other two countries are fortunate enough to have very robust institutions that keep governments in check. Israel has a very strong Supreme Court which can overrule the Knesset. Britain has the European Court of Appeals which can overrule Parliament, as well as the House of Lords and the world's oldest parliamentary tradition spanning nearly a millenium. New Zealand has none of these things. In New Zealand, our single-chamber parliament is supreme. A simple majority is all that is required to to wield unlimited power over the population. New Zealanders have no guarantee to human rights and free and fair elections that cannot simply be repealed by the government of the day.
Until recently, the customs and traditions of government in New Zealand, based on a Westminster model that dates back hundreds of years, was regarded as a sufficient safeguard for our way of life. But these traditions are now no longer universally regarded as sacred by our politicians. The Foreshore and Seabed Act, the abolition of Privy Council appeals, "Duynhoven's law", the Validating legislation, the Electoral Finance Act, and the repeal of Magna Carta rights in certain circumstances has shown our rights and freedoms cannot be trusted to our elected representatives. They must be permanently put beyond the reach of politics.

How Would a New Zealand Constitution Be Written?

Historically constitutions are painstakingly written and debated over by an elite for several years before being presented to the public fully formed. This is problematic, because New Zealanders may like some aspects of the proposed document, but disagree with others. A constitution is too important to be decided "in committee". It should be forged gradually, in public, over many years, and supported clause by clause by a two-thirds majority of voters at each stage.
All that is required initially is a shell constitution which does no more than establish the legal concept of a supreme law overruling all parliamentary legislation, and provide for its own amendment. It would look something similar to the following:
1. We, the People of New Zealand, incorporating all of its islands and territories, declare this document by mandate of majority popular vote to be the Constitution of New Zealand to act as Supreme Law of New Zealand. No past, present or future Act or resolution of Parliament, individuals or any assembled body, shall bind, alter or revoke this Constitution, except as specified herein.
2. Clauses of this Constitution may only be added, amended or revoked by resolution of a fairly held Referendum of all New Zealand citizens over the age of eighteen in which more than twice as many of the votes cast are in favour of the resolution as are opposed.
3. A Resolution to add to, amend or revoke clauses of this Constitution shall only be brought to Referendum by the Chief Justice of the Supreme Court of New Zealand upon presentation of:
a) a motion passed by the New Zealand House of Representatives, or;
b) a petition of no less than 50,000 New Zealand citizens over the age of eighteen.
upon which the Chief Justice must hold a Referendum within six months of, and no sooner than one month from, the date of presentation.
4. The Chief Justice may alter the presented wording of any proposed Resolution to reflect the intent of the original motion or petition with greater clarity, or to prevent ambiguity within the Constitution should the Resolution be successful, but shall not otherwise alter the original wording of the resolution or alter its intent in any way.
5. Final interpretation of the clauses of this Constitution shall be ruled upon by the Supreme Court of New Zealand. In cases where this Constitution's intent is unclear, or matters on which it is silent, priority in interpretation shall be given to New Zealand Law existing prior to the establishment of this Constitution.

Getting the Ball Rolling
The crucial factor in establishing a New Zealand Constitution is recognition by the courts. To do this, a fair referendum must be held where the constitution is supported by a majority of New Zealanders.
This could be done privately, with private money and with no assistance from parliament, at least in theory. But a constitution would likely be more legitimate if it was supported by initiating legislation. We need MPs and prominent people to support a single document written constitution for New Zealand, so we can make it happen.